dfdye said:
The ability to adjust the temperature set-point, temperature sensors can be had in everything from SMT packages to TO-220, so mounting them to the transformer becomes easier than finding room for a mechanical breaker. There is also the ability to do whatever you want with it. For example you could have it simply attenuate the volume instead of shutting the amplifier off.
Couple this idea with a PIC microcontroller for example, and you have yourself a very flexible and versatile form of thermal monitoring. One PIC could monitor temperatures for the transformer, rectifier diodes, and output stage heat sinks all at once. The possibilities are endless!
fitting a thermal sensor to the outside of a power transformer is almost useless.
The sensor must monitor the worst case temperature and that must be internally, not at the surface.
How would an amateur measure the worst case internal temperature and decide what surface temperature could be a suitable indicator of what was going wrong with the transformer or the duty it was being subjected to?
The sensor must monitor the worst case temperature and that must be internally, not at the surface.
How would an amateur measure the worst case internal temperature and decide what surface temperature could be a suitable indicator of what was going wrong with the transformer or the duty it was being subjected to?
AndrewT said:
Oh I'm glad you're here!
How small is too small?
At what point, or where, does one need to consider a larger transformer rather than additional capacitors?
How big is too big?
Oh, and here's a really wild guess on how big is too big, and I'm wondering if there's any validity, so fasten your seat belt
I try a very large transformer running such a small amplifier that the load is insignificant to the transformer. This wipes out the midrange. To compensate, I reduce the amount of capacitance. The midrange is still missing. So, I reduce the amount of capacitance again.
Soon, the smoothing is insufficient (buzz), yet the amplifier is otherwise playing within acceptable standards--although closer to a Fletcher-Munson LC than level, and dynamics are still small.
Is this a too-big transformer, or is it imagination?
P.S. Accurate location of max-rated temperature, according to Allied, is inside the transformer core, which isn't expected to work on spec if modified to fit a temp probe. Andrew is right. Allied also suggests to temp probe some other hot part of the device, such as an amplifier heatsink, because its output will be in proportion to the transformer that powers it.
This makes me wonder if a battery powered amp could have such consequences.
Too large capacity bank of battery's...
Too small...
Honestly, I don't believe it can be too big, if it's a stable voltage source which can deliver the power that's asked...
Maybe the caps are too big and would benefit if they would be paralleled with caps at 10 % and 1% the capacity of the largest ones. (might clear up the mids¿)
I'm confused
I need a hug
B.
Too large capacity bank of battery's...
Too small...
Honestly, I don't believe it can be too big, if it's a stable voltage source which can deliver the power that's asked...
Maybe the caps are too big and would benefit if they would be paralleled with caps at 10 % and 1% the capacity of the largest ones. (might clear up the mids¿)
I'm confused
I need a hug
B.
But... there have to be a limit when big is to big.
How about magnetising of the cores and raisetime to get the expected voltage, when working against a big capacitorbank ?
And how are we expecting the rectifyerbridge to behave ?
What happens if you put your PSB to a big industrial 100000VA transformer ?
How about magnetising of the cores and raisetime to get the expected voltage, when working against a big capacitorbank ?
And how are we expecting the rectifyerbridge to behave ?
What happens if you put your PSB to a big industrial 100000VA transformer ?
question
since a AB amp is about 65% efficient.
Is it correct to say 45% of your powersupply is turned into heat.
When you calculate your powersupply needs , don't you then at least
need to add that power loss into heat to your powersupply needs?
or is it just not as simple as that.
other problems are voltage drop at higher loads.
so with all of that a 150VA transformer is kind of borderline small for a 100W amplifier
since a AB amp is about 65% efficient.
Is it correct to say 45% of your powersupply is turned into heat.
When you calculate your powersupply needs , don't you then at least
need to add that power loss into heat to your powersupply needs?
or is it just not as simple as that.
other problems are voltage drop at higher loads.
so with all of that a 150VA transformer is kind of borderline small for a 100W amplifier
AndrewT said:
Well, I just tried that out. AndrewT is right.
I used TDA7924 shoving pairs of 8 ohm drivers (bit less than 4 ohms sum), a 50vct, 2 amper transformer, 100va, and 9023uF per rail.
Oh, and a very huge heatsink.
It made 100 very pretty watts and a lot of heat.
The 100va was fine.
With a party EQ'ing and some rather incredible concert like sounds. . . the power supply would hold up to the clipping point without a problem.
With a more like textbook-case power supply, th
jacco vermeulen said:
The cause of the 100w was mostly due to a stronger than normal load. Ah, this wasn't the manufacturer's "free and worth exactly that" design in their pdf, though. It was just a test of a new design. Either way, I think its supposed to be run at 50w.
Oh, what did it sound like? Well, that's not really the point during a stress test, but, okay. . . There wasn't any audible distortions. There just wasn't much difference, except louder and the heatsink got warm. It was nice--some form of hifi, but not quite top rank, and that seems like a typical comment about an ST amp. I much prefer the sound of a good BTL Tripath or a little LM1875, although the ST did nicely.
Well, we're on the power supply forum so. . . This TDA7294 had 220uF ceramic compound caps on the amp board instead of 2200uF electrolytic. There's a difference in heat at the heatsink. Does that mean there's a difference in efficiency?
I am sceptical for the need for a transformer VA greater than the maximum output power. Music is often assumed to be 10dB peak to avrage ratio. Therfore the avrage power is only a third of the maximum. What you are attempting to avoid is core saturation which will take a few cycles of operating above the maximum power rating of the transformer and I just don't think this will happen so easily. As for copper losses they should be not important for all sane sized capacitor banks, as the voltage of the capacitor bank sags it's conduction time increases. Personaly I go for about 0.8x max output rating in transfomer VA rating I shall do some maths to verify this assumption later however.
Core saturation is NOT caused by loading.
In fact, loading reduces Bmax slightly.
Cap input rectifiers have a power factor in the 0.5-0.7 range, or even less when using oversized transformers and capacitors, as many erroneously believe in. Not only does this mean you need a relatively large transformer, it also means you need a bigger power line circuit, very important for large amplifiers (>500W).
Tim
In fact, loading reduces Bmax slightly.
Cap input rectifiers have a power factor in the 0.5-0.7 range, or even less when using oversized transformers and capacitors, as many erroneously believe in. Not only does this mean you need a relatively large transformer, it also means you need a bigger power line circuit, very important for large amplifiers (>500W).
Tim
let's see the modeling and we can then comment more usefully.
If you are assembling a 68W into 8ohm chipamp and intend using a 54.4VA transformer, I would be very surprised to find that amp performance could not be improved, very cheaply, by increasing the transformer VA rating.
I am crazy with my 2x LONO 1000va each at 90vct ac ? no sir... I try it with 630va(already 4 time your and industry standard) under same chassis(exactly same voltage 4x 0-45ac),all from plitron No contest here, with 1000va it is magical and effortless, amplifier disapear ! not box at 130 pounds
But will follow AndrewT suggestion for better grounding !
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